Transformers are utilized in power transmission and distribution systems to modify the voltage of the power being provided. Being able to monitor and analyze various attributes about the transformer is critical for maintenance and troubleshooting as well as for proper and/or optimum loading. Various techniques are used to track the parameters of the transformer. Thermal stresses are a major factor in determining the lifespan of transformers. The operating temperature of the transformer has a major influence on aging of the insulation of the windings of the transformer.
Accordingly, there is a need to know the temperature (e.g., hot spot temperature) of the windings. The temperature of the windings may be simulated by measuring the temperature of the top oil of the transformer and then simulating the temperature increase for the hot spot. The temperature of the top oil may be measured using a capillary thermometer and a small heater may be used to simulate the temperature rise of the winding hot spot. Current from one of the bushings of the transformer is passed through the heater in order to raise the measured temperature. The heater requires calibration to remain accurate and is known to deteriorate with time. The capillary thermometer may provide a fairly accurate simulation of the hot spot temperature (e.g., within 2-3 degrees C.). However, for changes in temperature (e.g., increases) the capillary thermometer may lag behind the actual winding temperature in recording the changes (e.g., take 4 hours to reach direct winding temperature).
Electronic temperature monitors (ETMs) can also be used to calculate the temperature of the windings. ETMs may use resistive thermal devices (RTDs) that are more accurate (e.g., within 0.2 degrees C.) to measure the top oil temperature. The additional temperature rise of the winding hot spot over the oil temperature is added digitally. The ETM has the ability to tune the time constant of the ETM to match the time constant of the transformer (can adjust for different loads and thermal transients). Advanced ETMs may be able covert the temperature data into information that can be used for loading and/or maintenance.
Both the simulated and calculated temperature measurements are only predictions of the winding hot spot temperature. The measurements are only as accurate as the calibration information used to predict the temperature. Moreover, any change that may cause the windings to run hotter may not be detected.
Fiber optic probes may be used to measure direct winding temperature. However, the fiber optic cables and probes may be fragile and break. Moreover, the light source often has a limited life cycle. Replacing the bulb may require recalibration with sophisticated equipment and possible return to the factory. In addition the light source and the optical system may need to be calibrated so that they stay aligned.
What is needed is a more robust and maintainable system for measuring the direct winding temperature. Additionally, a system is needed for analyzing both predicted and actual temperature measurements for maintenance and operational purposes.